In Massive MIMO (multiple-input multiple-output) systems, base stations are equipped with a very large number (e.g., hundreds or thousands) of service antennas to simultaneously serve a number (e.g., tens or hundreds) of access terminals. Massive MIMO systems offer most of the benefits of traditional MIMO systems, but at a larger scale. In particular, Massive MIMO systems can provide high throughput, communication reliability, and high power efficiency with linear processing.
In conventional Massive MIMO systems, some access terminals may have similar channel vectors (e.g., due to being geographically close to each other), which may result in significant degradation in throughput for all access terminals. This is particularly true when max-min power control is employed. Max-min power control aims to maximize the minimum throughput, which effectively equalizes the throughput for all access terminals. The practical effect of such max-min power control is that the performance of the best performing access terminals are reduced in order to improve the performance of the worst performing access terminals. It has been shown that uniformly good service to all access terminals may be provided by selectively dropping a number of access terminals.
In order to determine which access terminals to drop, a conventional exhaustive search may theoretically be performed based on a calculated signal-to-interference-plus-noise ratio (SINR). However, this conventional exhaustive search is not practical because the decision of which access terminals to drop must be determined within a coherence interval, which is typically 1 to 2 milliseconds and the required calculations can be very complicated. For a large number of access terminals, the conventional search is impossible.